Of course, the philosophers' Problem Of The External World really demands a book. The topic certainly shouldn't be dispatched with the dogmatic brevity of an endnote. Yet stated baldly: irrespective of whether we are awake or asleep, what each of us intuitively apprehends as the mind-independent world "out there" - colourful, noisy and hugely refractory - is a virtual simulation run by one's own mind/brain. "The World" as apprehended beyond one's body-image is simply one simulation among billions of throwaway genetic vehicles spawned by selfish DNA. Each autobiographical virtual world is identical with distinctive patterns of neuronal firings in a vertebrate CNS. Thanks to the playing out millions of years of Darwinian natural selection, all but the most deranged mind/brains are coded to embody dynamic, data-driven simulations of their immediate environment. But such virtual worlds, like our conscious self, are no less fleeting, episodic and dispositional in their nature than are our beliefs and desires. In common with the conscious self, they disappear in a dreamless sleep.

The connection and activation weights of our neural nets, however, persist while their host organism slumbers. So "the world" abruptly recreates itself when we "awake". Opening one's eyes serves to re-impose selective discipline on our ways of worldmaking [in the proximate, non-Darwinian sense of "selective"]. Thus on waking up each morning, one's capacity to generate a virtual world becomes constrained once more by inputs from the optic nerve. The austere regimen of one's episodes of waking life contrasts with the psychotic excesses of one's dreams.

On this interpretation of the Human Predicament, one can only ever infer that there are billions of other experiential worlds like one's own. Indeed one can only ever infer the existence of a vast natural Multiverse - whose organisms and virtual worlds play host to unfolding virtual dramas like one's own. Likewise, one may infer that the contents of one's virtual world are tightly selected by peripheral impulses when one is awake. Conversely, when one is asleep, or hallucinating in a sensory-deprivation tank, or tripping on major psychedelics, the features and narratives of one's virtual world are quasi-autonomous. One tends to get "lost in space" - an abstract neural weight-space of possible worlds.

"Waking up" does not turn the neuronal firings identical with one's colourful dreamworld into a neutral vehicle for the occult faculty commonly known as "perception". One's nerve cells don't - indeed can't - metamorphose into a transparent medium for accessing the extra-cranial Real World. Likewise, when "awake" rather than dreaming, one doesn't cease thinking and talking in mentalese masquerading as a public language. This feat would be pure ontological magic. It would also wantonly violate Occam's razor. Mentalese is the only language one can ever know.

In the course of evolution, natural selection has churned out billions of species-specific virtual worlds - i.e. rival organic
quantum supercomputers - in creatures with central nervous systems. The simulations run by such virtual worlds serve as disposable genetic vehicles no less than the organisms who host them - and whom they help reproduce. Like their hosts, these virtual worlds senesce and die. Some macroscopic worlds are fitter than others. Host organisms whose brains run such genetically adaptive virtual worlds tend to leave more copies of themselves and their kin than their genetic rivals.

A cardinal feature of each virtual world is its egocentricity.
Each of us lives in a world whose centre is our body image. Virtual worlds are egocentric because coding for a self-centred universe helps maximise the inclusive fitness of selfish DNA. A "view from nowhere" would be genetically maladaptive. So world-making DNA macromolecules ensure that the egocentric delusion is a heritable design feature of the worlds they encode.

The virtual worlds of the Darwinian Era may be classed as "organic VR" because their contents are in part selected (but not created) by organic peripheral inputs. These electrochemical impulse-patterns are themselves in turn shaped by sensory transducers at the body surface. "Silicon VR" or, more generally, "synthetic VR" refers to virtual worlds whose selection-regime of inputs derives directly from non-organic retinal imaging devices, body-suits, silicon implants, etc.

To complicate matters, there will soon be artificial bionic devices that blur the distinction between organic and synthetic VR. Moreover, unless silicon systems support the warm QM-coherent states needed for experiential manifolds - a most unlikely proposition - there is a sense in which all experiential VR is organic. For it inheres in organic wetware alone [on account of the unique valence properties of the carbon atom]; only the mode of world-selection is different.

Evolution has harnessed the intrinsic properties of certain minds/brains/virtual worlds to play a representational / computational / simulational role in the organisms it spawns. This process of recruitment occurs because near-real-time tracking of regularities in the local environment is genetically adaptive. It allows awake bodies to navigate a dangerous world.

But unreflective naïve realism is itself a highly adaptive delusion for organisms in its grip. The mind-independent world doesn't - and couldn't - directly imprint its signature on our brains/minds/virtual worlds. Their intrinsic properties are not - and couldn't be - contingent on the particular occasions on which they are triggered. [Actually, this is an over-simplification. The separability and individuality of events in our classical worlds may emerge from the non-locality (but see Mike Price's Everett FAQ) and superposition of pluralities of its fundamental quantum substrate. This is a big subject.]

The delusiveness of perceptual realism will be clearer when we are able to construct minds/brains/virtual worlds to order in vats; or sooner still, when immersive multi-modal VR becomes a trillion-dollar entertainment industry late this century. Notoriously, the dominant technology of an era tends to supply its root metaphor of mind; and the advent of pervasive VR will probably transform our root metaphor of mind into some sort of virtual world. In any event, although "synthetically"-selected, none of the states of our future virtual worlds will be either more or less natural, nor inherently more or less representational, than others. This (non-)representational status is context-dependent. The most that any proximate selection-process can ever do is to ring the changes on a pre-set menu of neurochemical pathways. For just as there are a finite number of games of chess, there are a finite number of mind/brain/virtual world states for a system of any given size. But we've scarcely begun to explore them.

It's worth briefly contrasting the inferential realist perspective sketched above with the Dennettian argument that conscious mind, insofar as such a phenomenon exists at all, is a virtual serial computer supervening on
a parallel one.

Unfortunately, the Dennettian approach confuses conscious mind with self-conscious mind and its thought-episodes; and relies on a crude realism about "perception".

In reality, the most intense experiences one undergoes (e.g. "physical" agony) are also the most "primitive". One's stream of thought (including "encephalised" emotion) may indeed be akin to a serial computer supervening on a parallel one. But it is parallel computation which embodies the most intense and vivid modes of consciousness, whereas the consciousness of the virtual serial computer which supervenes upon it is phenomenologically impoverished. Thus introspecting one's thoughts is hard work at the best of times. It is extraordinarily difficult even to count or individuate them. Cognitive phenomenology is rarefied and subtle. By contrast, one's visual, auditory and tactile worlds - whether or not they are cross-modally matched - are vivid, incontrovertible and extraordinarily intense; and these virtual worlds go a long way down the phylogenetic tree.

If you think you're plugged straight into the Real World, then the prospect of plugging in to silicon VR will seem like a retreat into fantasy-world escapism. On the other hand, if you've long ceased to believe that The World was yours to lose in the first place, then you may decide that nasty old organic VR is a world well lost.

FURTHER READING
Perhaps the best contemporary treatment of the inferential realist perspective can be found in Steven Lehar's
The world in your head (2003), and Inner Presence
Consciousness as a Biological Phenomenon (2005) by Antti Revonsuo